Lieutenant CommanderData is commonly in charge of the night shift, but he is out on an away mission. A deep space probe containing radioactive material has crashed on a planet, Barkon IV, hosting a preindustrial civilization. Due to Data's android nature, he is the one best suited for the mission. He is hailed by the Enterprise's bridge by EnsignRainer, but no response is received by them. Dr. Crusher is not overly concerned, as the radioactive nature of the fragments Data is supposed to collect on the planet are expected to disturb any received and sent messages, as Geordi La Forge had warned.

Crusher says that Data can wait for a few days before being picked up by the Enterprise as the ship has to rendezvous with the USS Lexington to deliver medical supplies to the Taranko colony.

In a village on Barkon IV, two Barkonians, a middle-aged man Garvin and his young daughter, Gia, are talking. They are interrupted when Data walks in, his appearance suggesting that he has been through some rough experiences; his hair is tousled and his uniform is dirty and ripped in places. He stops when he sees them, but is only able to emit non-vocal sounds.

Garvin tells his daughter to go home and tries to talk to Data, but he at first can only repeat what Garvin has said. It is a slow process talking to him, but Data eventually recovers enough to state that he can explain he has no idea who he is. His only memory is that he walked to the village from the mountains outside it, which Garvin remarks is very far away, over 200 seltons. Garvin sees that Gia only hid behind a wooden wheel and again makes her leave for school. She is not afraid of Data.

Garvin points to the box Data is carrying, asking what it is. Data is not sure, and hands him the box, the weight of which surprises Garvin. Garvin asks what the writings on it mean, and Data immediately replies "Radioactive", though he cannot say what the word means, and suggests that it is his name. Garvin opens the box in the hopes of finding some clues as to Data's identity, but finds only several hunks of silver metal, one of which he holds in his bare hands and examines closely. It is obviously harmful, but neither of them know it.

On the Enterprise, Troi asks CommanderRiker in his quarters if she is allowed to take the Bridge Officer's Test. Riker wonders why, and is told that she has considered it off and on for over two years, but was motivated by her class reunion, as well as being in charge of the bridge when the Enterprise-D struck a quantum filamenttwo years before. Riker says she has his full support, though he will be involved in evaluating her.

Garvin and his daughter take care of Data because of his amnesia. Because they do not know his name, they call him Jayden, after initially having suggested that his name might be "Radioactive" since it was written on the case. Talur, a scientist and doctor, takes a look at Data and proclaims that he is not ill despite the amnesia, and that his pale appearance is because he is an Ice-man, a humanoid native to the mountains outside the village. Data is dressed in Garvin's old clothes and taken to the village's blacksmith, Skoran. Skoran shows interest in the metal fragments Data had collected and offers to buy half of them. There is also an incident at Skoran's and an anvil falls on a man's leg after the support gives way due to rotting wood. The villagers cannot lift it, but Data does so with ease, shocking everyone.

At dinner, Talur posits that Data's strength is something he shares with all Icemen from the Vellorian Mountains, that they have to use their strength to fight off the wild beasts that roam the mountains. Data pushes that claim, saying she also said no one has ever seen one of these creatures. Then, Garvin finds himself feeling tired; Talur tells him to take a walk and get some fresh air to feel better. When he's gone, Gia notes that Data didn't eat, asking if the food was not to his liking, which Data denies, commenting that he simply didn't have an appetite. Gia comments that her father's cooking isn't as good as her mother's, revealing she has died. She tells Data that her father says she went to a place where no one gets hurt or sick, and asks Data if he thinks a place like that truly exists. Data looks out the window towards the stars and states that he is certain it does.

In engineering on the Enterprise, Geordi La Forge and Worf are the senior officers in Troi's test simulation, where the insulation of the antimatter storage pod chambers is breaking down. Troi has to make fast decisions, but fails to stop the inevitable warp core breach. Riker tells her that all parts of the test went well for her, except for that last and most important part of it, resulting in Troi failing the whole test. Troi asks what part she failed, but Riker says he cannot tell her; not knowing is part of the test.

On Barkon IV, Data follows Gia to Talur's school. Talur teaches the Barkonian children about the elements: sky, fire, rock, and water. She says that wood is combustible as it contains fire and, when exposed to a flame, the fire within the element comes to life. When the fire is put out, air is released as smoke. Data is not convinced that this philosophy is correct. He voices his opinion that he knows fire is not an element, and that wood is composed of many different chemical compounds of which none is fire. Talur shakes Data's words off, telling the children that his amnesia makes his reasoning unreliable. After Talur and the other students leave, Gia asks Data if he thinks Talur is correct. Data responds that, despite his amnesia, he is convinced that fire is not an element.

Just then, they overhear Garvin arguing with Skoran about the price of the metal that changed hands. Skoran is trying to cheapen the deal, blaming Garvin's memory. Suddenly, Garvin begins to feel weak and ill, and has to go home. Talur examines him, but does not know how to handle Garvin's symptoms, involving hair loss and third-degree burns all over his body. She gives Gia some herbs, and hopes that they will help Garvin to recover. Data is not satisfied with the treatment Talur suggested, so he starts to investigate Garvin's illness more closely. He and Gia go to the market to get supplies to make equipment for his investigation, and there are confronted by Skoran and several others. They blame Data for bringing a sickness to the village, leading Data to realize that Garvin's sickness has spread. The townsfolk angrily tell Data to leave the village as he and Gia leave the market quickly.

Data constructs the equipment he needs, including a five-hundred power magnifying glass and a Geiger counter. He studies cell samples from both Garvin and Gia, who also has started to show symptoms of radiation sickness. Talur comes with suggestions, that maybe Data is the one to have brought the sickness to the village, but it is only after Data has examined a pendant of Gia's more closely that he comes up with the cause for the villagers' sudden illness. The pendant was made by Skoran from the radioactive metal, and a large piece of the metal is the pendant itself. This also explains why the blacksmith has fallen ill as well.

On board the Enterprise, Troi is intensely studying the ship's technical specifications when Riker enters her quarters. He tells her that she has failed the Bridge Officer's Test three times, and that she cannot take it again. She protests and says that she will take it as many times as she has to in order to pass it. She tells him there's more to being a bridge officer than studying technical manuals. Riker tells her that she shall not take the test again, that she does not possess the qualities expected from an officer on the bridge. He also says that no matter how much he likes her, his first duty is toward the ship. His last words before his exit make Troi think, and she rushes to the holodeck.

After having entered the holodeck, Troi starts the test's simulation again. She is in engineering, and the antimatter is about to leak out of its container. She asks La Forge if he can repair the damaged parts from inside a crawl way. The Worf hologram explains that La Forge will not survive the radiation within, but Troi, well aware of this, orders the holographic La Forge to repair the damage anyway. As an anguished Troi watches him enter the crawl way, Riker, having known she would not give up so easily, enters and congratulates her on passing the test. She immediately deduces that the point of the test was to see if she could order someone to their death, which Riker confirms. Having known this was part of being in command, but having still hesitated when the time came, Troi admits that he may have been right about her being ill-suited for the job—to which Riker counters that she passed with flying colors; she had tried every possible solution she could think of, and when nothing else worked, she made the painful but necessary decision required to save the ship and has therefore earned her promotion to Commander.

On Barkon IV, Data places the metal before the indicator of radioactivity, and discovers that the metal is the cause of the sickness. The indicator consists of a wooden frame and a linen cloth. The cloth has been treated with a liquid Data took from a lamp in Garvin's house. When exposed to a candle, the cloth illuminates, and when the metal is present, the cloth is illuminated by green dots scattered over the surface. This tells Data that the metal emits invisible particles, which however can be blocked by the box he brought to the village. He tells Talur to collect all of the metal fragments in the box, while he tries to find a cure.

Skoran enters the house later, together with another villager. They attack Data and tear off a piece of skin from his head, exposing his internal circuits and flashing lights. The two villagers become scared and ask him what he is, but Data cannot answer the question due to his amnesia. The villagers retreat, scared, and Data is left in a confused state.

Data has come up with a cure to the radiation sickness. He gives it to Gia, telling her that it worked on her father's cellular samples and that her father is recovering. Data says that he has to give it to the entire village, but that they never will trust him due to his exposed android nature. Gia offers to help, but is told that she will not be trusted either. As the village's well is the only water source, since the nearest river is two days away, Data decides to pour the cure into it. At night, Data exits Garvin's house and reaches the well. As he is about to pour the cure into the water, the villagers appear. They say that it was Data who made them sick, that he brought the plague to their village. Data pours the cure into the water, but is then stabbed in the back by Skoran with a long pole. The blacksmith is shocked with electricity and Data collapses onto the ground, the pole going through his torso.

Sometime later, Gia stands before a tombstone with Barkonian letters. Cosmetically altered to appear Barkonian and dressed up in the local style of clothes, Crusher and Riker approach her, asking her if she has seen a friend of theirs, pale with yellow eyes. Gia nods and speaks about Jayden in past tense. Crusher wonders why, and is told that Data is dead, and that the tombstone is his. Gia says that she did not know his name, so they called him Jayden, and wonders what his real name was. Gia also says that Data was killed because he was different and the villagers were scared of him, despite his efforts to cure them of the sickness the strange metal fragments caused. Riker wonders where the fragments are and is told that they are buried in the woods. After Gia has left, Riker suggests that Data can be beamed up from his resting place directly onto the ship, together with the metal fragments.

In the Enterprise's sickbay, Data is repaired and put on-line again. CaptainPicard asks him if he remembers what happened, and Data says that he was recovering the information from the probe when he was shocked by it. He suggests that it overloaded his positronic net and that, as a result, he has no memory from that moment on. He says, though, that his clothes suggest that he had been through quite an adventure. Troi also tells Data that she will be his superior officer from that moment forward, as she was promoted during Data's absence.

"You know, this is a much better way of communicating for you. It's far less confusing than the way you normally speak."

- Troi, to Riker when he uses his trombone to talk

"Did you come here for something in particular, or just general Riker-bashing?"

- Riker, to Troi after picking on his communication

"Congratulations, you just destroyed the Enterprise."

- Riker, to Troi after a failed Bridge Officer's Test

"Mr. Radioactive?"

- Garvin, trying to find a name for Data

"Stay out of this, iceman!"

- Skoran, to Data

"The word "radioactive" may be a warning about the dangerous nature of the metal."

- Data, to Talur

"What... what are you?!"
"I do not know."

- Skoran after the "skin" of Data's face is ripped off and his android inner workings are exposed

"We must find him and stop him before he kills us all."
"I don't... not Jayden. He wouldn't try to hurt us."

- Skoran and Garvin, referring to Data

"Skoran said you're some kind of... creature."
"I do not know what I am... but I am not like you."

- Gia and Data

"Jayden... why are you wearing that hood?"
"I do not wish to frighten you."

- Gia and Data, hiding his face from her

"Where is your mother?"
"She died about a year ago. Father said she went to a beautiful place, where everything is peaceful and everyone loves each other and no one ever gets sick. Do you think there's really a place like that?"
"Yes. I do."

- Data and Gia, discussing her mother's death

"Tell me one thing – is there a solution? Or is this simply a test of my ability to handle a no-win situation?"
"There is a solution..."
"Then give me time to find it!"
"I can't – as much as I care about you, my first duty is to the ship. I can not let any bridge officer serve who's not qualified – I'm sorry."

- Troi and Riker, after he cuts off her attempts to pass the Bridge Officer's Test

"My first duty is...to the ship... the ship!"

- Troi, realizing that Riker had hidden the solution in his previous comments

"Congratulations, you passed."
"That's what this was all about, wasn't it – to see if I'd order someone to their death?"
"That's right..."
"I knew that was part of being in command, and I thought I'd prepared for it – but when the moment came, I hesitated...Maybe you were right, maybe I'm not cut out for this..."
"You did exactly what you had to do. You considered all your options, you tried every alternative, and then you made the hard choice."

- Talur and Data, discussing the radioactive metal fragments, and inadvertently invoking his actual name

"I was attempting to download the sensor logs from the probe's on-board computer. There was a power surge... I believe the surge overloaded my positronic matrix. After that, I have no memory until this moment... (seeing himself still in Barkonian clothing)... but it appears, I had an interesting time."

- Data, after returning to the Enterprise

"Well... if you'll excuse me, um... I have the bridge this watch."
"Counselor, have you been promoted in my absence?"
"That's right. Which means from now on you can call me "sir.""
"Yes, sir."

Budget requirements necessitated a ship-board B-story to fill out the show. Moore decided to avoid a predictable "We've lost Data and we got to find him!" plot in favor of the story of Troi's promotion, which had been originally conceived for "Liaisons". He explained, "I thought that was a neat idea and was a good move for the character. The whole notion was something I wanted to do ever since I read Jeri Taylor's novelization of "Unification", where there is a line about Troi reflecting on her experiences in the episode "Disaster", where she got command temporarily. Jeri had a line in there about tasting blood and wanting to again, and that stuck with me. I thought that was an interesting direction to take Troi." (Star Trek: The Next Generation Companion; Captains' Logs: The Unauthorized Complete Trek Voyages)

In discussing her considering becoming a commander with Riker, Troi references the events depicted in TNG: "Disaster", wherein she found herself the highest-ranking officer on the bridge after a quantum filament hit the ship and disabled all communication and transportation on the ship, and clearly found the task of taking command overwhelming.

Troi asks Riker if the engineering qualification portion of the Bridge Officer's Test is "a test of my ability to handle a no-win situation". She may be thinking of the well-known Kobayashi Maru scenario formerly given during officer training at Starfleet Academy, although her test does turn out to have a solution.

On Troi's promotion, Taylor remarked, "I thought it was really strong, although we have taken some criticism from people who said, 'How could you promote her over Data and Geordi?' But it would not have been a very interesting story to see Geordi or Data getting their rank. The obstacles she had to overcome were formidable and where you get interesting drama is out of conflict" (Captains' Logs: The Unauthorized Complete Trek Voyages)

Moore admitted having mixed feelings on the episode. "As a writer, I never figured out what it was about. I didn't know what I was trying to say with the episode. It was probably the most difficult writing experience I had on the show because I was very frustrated. It was a bad time in the season. I was tired and I was not having fun, and I think it showed in the writing...What I enjoyed writing was Data as Mr. Wizard on the planet of people who aren't very smart. That was kind of funny. I got a kick out of Data being the guy in the back of the class raising his hand, inventing quantum mechanics with stone knives and bear skins." (Captains' Logs: The Unauthorized Complete Trek Voyages)

Brannon Braga commented, "It was great to see an episode as fundamental as Data being forced to confront his inner nature. The cool factor was also very high. Seeing Data impaled by a metal rod was just great. I enjoyed the episode. A couple of the performances of the townsfolk were a little groggy for my taste, but Data as Frankenstein – what a neat idea. I thought, all in all, it was a very nice job. In fact, I thought it was probably one of the best ones of this season." (Captains' Logs: The Unauthorized Complete Trek Voyages)

Director Winrich Kolbe observed, "It was interesting because I had done "Pen Pals", a show that didn't quite find the right direction. Data seems to have a predilection for little girls, not in a bad way. Here we wind up again with a little girl. There seems to be something about Data that gives writers the idea for the relationship. He has a vulnerability or an innocence, especially at the beginning. It's Brent at his best. I liked the episode. We got some good actors and it was almost like a period piece. It was like going back to the Renaissance." (Captains' Logs: The Unauthorized Complete Trek Voyages)